Wave modulation



Aug. 16,1927. 1,639,000

J. W. HORTON WAVE MODULAT ION Filed Jan. 4, 1924 l/zwnfar: Jasep/r N liar/on.

' Patented Aug. 16,1927.

UNITED STATES, PATENT, Ol' FICE.

JOSEPH W. HORTON, F IBLOOMFIELD, NEW JERSEY, ASSIGNOR TO WESTERN ELECTRIC COMPANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK,

' WAVE MODULATION.

Application filed January 4, 1924. Serial No. 684,305.

A feature of the invention. comprises the use of a condenser to effect wave modulation .by capacity variatlons.

A relatedjea'ture comprises a condenser of special type for obtaining the desired characterof wave to be modulated.

Briefly and specifically, the invention makes use of the principle that in any' cirb cuit havingfcapa city, the current in the cir; cuit is afunction-of the capacity and ts wave form can be principally determined by the capacity variations if the other constants of the circuit remain substantially the same. In the circuit accordin to the inventionthe constants are prefera l'y such that the circuit is nonr es'onant'or aperiod c.

The capacity may be embodied in a suitable type of condenser which is variable either at the frequency'of the wave to be modulated or of the modulating or control wave. If a source of potential variations current in the circuit, the impedance variations being in accordance with the wave variations used to controltheconderer; The various objects and features ofv the.

invention will appear more fully from the following detailed description of certaln embodiments disclosed in the accompanying drawing.

' Fig.1 is a schematic representation of a modulating circuit according to the invention, in which the capacity is varied at the carrier frequency; v

Fig. 2 is a similar representation of a circuit employing, a difierent type of condenser;

Fig', 3 gives curves illustrative .of the de- Slgl of the condenser employed in Fig. 2; an

Fig. 4 shows -a cireuit in which ,a capacity is varied at speech frequencies to modulate h a carrier wave. v

Referring first to Fig. 1, the condenser ing supplied with driving energy from the is connected into the circuit, the condenser, 1 offers a varying impedance-to the flow of P ance of the tube, -erally of such high .and 11 and the on the condenser does mediately but persists for a few periods of illustrated in this figure isshown as comprising a stationary plate 1 and a movable plate 2. The movable plate is in this figure in the form of a vibrating bar of diaphragm mounted on suitable fulcrums or bearing points 3 and As shown, the bar is supported for free flexural vibration, the fulcrums being substantially at the fundamens f tal nodes, which, as is known, should be located at points 0.22418 of the length of the bar from its ends. A magnet 5 may be employed for maintaining the bar in=a state of vibration, this magnet being energized from any suitable source '6 of waves of the frequencyto be modulated. This source may e a separate unit such as a vacuum. tube oscillator, or the magnet 5 may have a windoutput of amplifier A so that the system including magnet 55, condenser 1, 2-and tube A form a regenerative circuit. 7

The condenser 1, 2 is connected in circuit with the secondary 7 of a speech transformer, and preferably a vacuum tube amplifier A. The output of the amplifier may be connected to any suitable load circuit L.

Speech currents from the microphone cir cuit8 set up potential variations across the terminals of the secondary 7. These pententialvariations energize the series circuit including the condenserl, 2.and the input circuit of the amplifier A. This input circuit is shown as comprising the resistances 10 l and "11 shunted by the grid-filament imthis latter being genorder that it may be an open circuit. The be inserted for polarizing the grid and the blocking condenser 13 may be made sufficiently large to offer negligible impedance in the series circuit.

The pontential variations in the coil 7 set up current. flow in the closed circuit, as

considered practically battery 12 may above traced, including the condenser 1, 2-

and the resistance in shunt of the grid and filament of the tube A. This current charges the condenser 1,' 2, and if the resistances 10 10o frequency of vibration of the bar 2. have such values that the charge not leak off imvibration of the bar 2, therewill be a high .105

frequency voltage set up across the terminals of the condenser 1, 2. If the potential across the terminals of coil 7 were constant, there would be a voltage wave developed between the terminals of condenser 1, 2 having a frequency equal to the frequency of vibration of the bar 2 and having a constant maxipotential of the grid of the tube A with respect to its filament. Due to the well known action of the device A, amplified waves of the same form as those impressed on the grid are transmitted into the load circuit L. By closing the switch 14: an additional capacity 15 may be included in parallel with.

the condenser 1, 2 to increase the total capacity of the'circuit. When the condenser 15 is in circuit, the percentage change in capacity produced'by a given variation in the capacity of the condenser 1, 2 is smaller than when the condenser 15 is omitted. The degree of modulation produced may therefore be controlled b the amount of capacity included in paral e1 with the condenser 1, 2.

In Fig. 2, the portion of the system to the right of the broken vertical'line may be consideredidentical to that shown in Fig. 1 to the right of the corresponding broken line. In Fig. 2, a somewhat different type of variable condenser is shown from that of Fig. 1. This condenser includes one or more fixed plates 16 and'one or more'rotatable plates '17 mounted on the shaft 18 and arranged to be continuously rotated by means of the motor 19.

The condenser 16, 17 is preferably constructed so that by rotating the plate 17 at a constant speed with respect to the plate 16, sinusoidal variations in capacity are produced. This may be accomplished by using a plate 16 having a straight edge which passes through the axis of the shaft 18, and

by fashioning the plate 17 in the manner more fully indicated in Fig. 3.

In this figure the dotted line is a circle of radius 1' and is defined in polar coordinates as B=sin 9 The shape to be given to the plate 17 is indicated in Fig. 3 by the solid curved line 20,

and its contour is defined by the equation R :sin 9 where/ fr has the same value as in the previous eguation. If a plate of this shape is mount: e on the shaft 18 so that the origin in Fig. 3

coincides with the axis of the shaft, then Y sults.

condenser 16, 17. varies sinusoidally according to the relation' O=C(l cos pt) where Crepresents the instantaneous capacity and C is the average capacity, 79 being equal to the frequency of rotation multiplied by 271'. A condenser of this type has the advantage not only of producing sinusoidal voltage variations, but of permitting large changes in the capacity. since any desired size may be given to the plates, and also a condenser may be built up from a number of plates as in the case of the usual variable air condenser. It will be understood that the operation of the circuit of Fig. 2 is essentially the same as that indicated in Fig. 1. The movable plate-17 is rotated at the carrier frequency, and when speech currents are set up in. the circuit 8, a wave of the carrier frequency modulated by speech is produced and transmitted.

In Fig. 4, the variable condenser is controlled by speech waves and in turn controls the amplitude of carrier waves impressed on the circuit from a carrier frequency source. The carrier source is indicated at -21 and may be of any suitable type, such as a vacuum tube oscillator. The carrier waves may be amplified by the vacuum tube amplifier A and the resulting amplified waves impressed on the modulating circuit through the transformer 22. \Vhen speech is directed against the diaphragm 23, which forms the movable plate of the condenser 23, 24, the impedance of the series circuit including the secondary of transformer 22, the'condenser 23, 24 and the primary of the output transformer '25, is varied in accordance withspeech variations so that a variable amplitude wave re- The modulated wave is then transmitted to the load circuit through the filter F, suitable amplification being also supplied if desired.

The series circuit including the variable condenser in each of the figures is preferably substantially aperiodic so that the modulating action is carried out independently of any resonance effects and depends principally upon the capacity reactance of the variable condenser.

Insteadv of producin the capacity variations "as in each of the gures by moving one of the plates ofa condenser, these variations may be effected by moving a dielectric. For 4 example in Fig. 2, the number 17 may be of dielectric material, rotatable between a pair of condenser plates 16 connectedin the circuit. Also in Figs. 1 and t the vibrating member may be of dielectric material positioned between-the condenser plates.

Various other modifications of the modulating systems illustrated in the drawing will occur to those, skilled in the art, the

scope of the invention being defined in the aperiodic circuit, and means to control said capacity in accordance with wave variations to be transmitted. v I

2. In a modulating system, a substantially aperiodic circuit, means for causing a flow of current therein responsive to a variable voltage impressed thereon from a source external to said circuit, a variable capacity in said aperiodic circuit, and means to control said capacityin accordance With wave variations whereby a variable amplitude wave is pro duced, the amplitude of said wave being sub stantially independent of any resonant eflect in said circuit. i I

3. In a transmitting system, a circuit, a source of signal voltage variations therein, a condenser in series with said source, and means for varying the capacity of said condenser at the carrier Wave'frequency to produce a signal-modulated wave;

4. In a transmlssion circuit, a condenser 0 having one plate movable with respect to 1 another plate, means'to move the movable plate at the frequency of a carrier wave, and a source of signal voltage variations in circuit with said condenser for setting up variable current 'flow in said circuit, whereby a modulated carrier wave is produced.

5. In a modulating system, a vacuum tube having a cathode, a grid and an output electrode, a resistance connected across the grid and cathode, a source of signaling voltage variations and a variable condenser in series with each other and said resistance, means to vary the capacity. of said condenser at the carrier wave frequency whereby a modulated Wave is produced, and an outgoing circuit coupled to saidoutput electrode.

6. In a .modulator circuit including an electric discharge tube having a cathode, and agrid or the equivalent; a series circuit including said grid and cathode, a source of signaling potential variations, and a capacity; and means to vary said capacity at the carrier wave frequency to produce a signalmodulated wave.

7. A modulator-circuit as claimed in claim 6, said capacity and themeans for varying the same being of a type to produce sinusoidal variations in capacity.

In Witness whereof, I hereunto subscribe my name this 3 day of January, A. D., 1924.

Josnrn W, HORTON. 

